<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
     <script>
      let grades=+prompt("输入成绩")
      if(grades>=90){
        console.log('A')
      }else if(grades>=80&&grades<90){
        console.log('B')
      }else if(grades>=70&&grades<80){
        console.log('C')
      }else if(grades>=60&&grades<70){
        console.log('D')
      }else{
        console.log('E')
      }
     </script>
</body>
</html>